Giuliani called Michael Cohen a 'pathological liar' in a heated CNN interview, and it could create another problem for Trump

• Rudy Giuliani called Michael Cohen a "pathological liar" in a heated CNN interview.

• It came after Cohen reportedly said Trump approved an infamous meeting between campaign staff and Russians during the 2016 campaign.

• Calling Cohen a "pathological" liar suggests that he did it all the time — including when he worked for Trump in the past.

• That would create another headache Trump's defense team.

• Giuliani went for Cohen during the interview, calling him "an incredible liar, who has a tremendous motive to lie."

Rudy Giuliani called Michael Cohen a "pathological liar" in a heated respond to reports that Cohen had claimed that Donald Trump personally approved a meeting between his campaign staff and representatives of Russia.

However, the fierce criticism could create a new legal problem for Trump, given that Cohen worked as his personal lawyer for 11 years.

Giuliani's comments came after Cohen reportedly claimed that he was there when Donald Trump Jr. informed Trump of a Russian offer to share dirt on Hillary Clinton .

Trump then approved a meeting taking place, CNN reported on Thursday night, citing sources with knowledge of the matter.

Cohen is reportedly willing to make those claims to Special Counsel Robert Mueller, who is investigating the Trump campaign's ties to Russia.

Giuliani, Trump's defense lawyer, launched a barrage of personal attacks on Cohen in response, calling him a "pathological liar" and "an incredible liar, who has a tremendous motive to lie because he's got nothing to give."

You can watch Giuliani's comments here:

He told CNN's Chris Cuomo on Thursday night: "I expected something like this from Cohen. He's been lying all week. He's been lying for years.

"The tapes that we have demonstrate any number of very serious lies by him back a year and a half ago."

"There's no doubt in my mind that he's just not credible. I would not accept him in a case as a witness, as a prosecutor," he added later.

"This is the kind of witness that can really destroy your whole case, because any finder of fact loses confidence in the case when you rely on a guy like this. When you rely on a guy where, before you're finished with this, you're going to have such a string of lies you just can't trust him."

But in calling Cohen a liar, Giuliani may have created another legal problem for Trump — by effectively asserting that Cohen had been lying for Trump while still working for him. Cohen worked for Trump from 2007 — at the time fighting the then-real estate mogul's legal battles with his residents — until just a few months ago.

Rep. Eric Swalwell, the top Democrat for the CIA on the House Permanent Select Committee on Intelligence, told CNN on Thursday : "Rudy Giuliani described him [Cohen] as a proven liar but all of his lies in the past were on behalf of Donald Trump."

Giuliani added in his Thursday interview that he didn't "see how he [Cohen] has any credibility," referring to Cohen's lawyer's releasing a tape earlier this week of what appeared to be a secretly recorded conversation between Cohen and Trump.

In the tape, the two men discuss how to buy the rights to a former Playboy model's story of her affair with Trump.

Cohen is the third Trump associate to suggest the president knew more about the 2016 meeting in Trump Tower than he's letting on.

Giuliani previously told Business Insider he "would be surprised" if Trump knew about the meeting at the time that it happened. But he also left open the possibility that Trump might have known but later forgot about it.

The Thursday report of Cohen's claims is further proof that Cohen could flip on Trump in the Mueller investigation — which would be a nightmare for Trump's legal team.

"Anyone that takes care of problems and buries the bodies is not someone you want to testify against you," former Justice Department prosecutor Jeffrey Cramer previously told Business Insider's Sonam Sheth .